Buggered if I know. :-) But in my experience of singing along to them, "carol service" generally involves a dose of sermons in your sing-song, whereas "carol concert" would involve singing but not sermonising.

My personal feel is that if you sing Christian songs then it doesn't automatically make the performance religious - think David Bowie doing "Little drummer boy", or a whole bunch of country singers. Something needs to differentiate between a Christian song as entertainment and a Christian song as praise. But anyway, that's only marginally relevant to the thread.

Graham.